1. Field of the Invention
The present invention relates to certain substituted guanidines, and methods of treatment and pharmaceutical compositions that utilize or comprise one or more such guanidines.
2. Background
A number of substituted guanidines have been reported. See, e.g., U.S. Pat. Nos. 1,411,731, 1,422,506, 1,597,233, 1,642,180, 1,672,431, 1,730,388, 1,756,315, 1,795,739, 1,850,682, 2,145,214, 2,254,009, 2,633,474, 3,117,994, 3,140,231, 3,159,676, 3,228,975, 3,248,426, 3,252,816, 3,283,003, 3,270,054, 3,301,755, 3,320,229, 3,301,775, 3,409,669, 3,479,437, 3,547,951, 3,639,477, 3,681,457, 3,769,427, 3,784,643, 3,803,324, 3,908,013, 3,949,089, 3,975,533, 3,976,787, 4,060,640, 4,014,934, 4,161,541, 4,709,094, 4,906,779, 5,093,525, 5,190,976 and 5,262,568; PCT applications WO 90/12575, WO 91/12797, WO 91/18868, and WO 92/14697; and H. W. Geluk, et al., J. Med. Chem., 12:712 (1969).
The amino acid L-glutamate is widely thought to act as a chemical transmitter substance at excitatory synapses within the central nervous system. Neuronal responses to glutamate are complex and appear to be mediated by at least three different receptor types, i.e., KA, QA and NMDA subtypes, each being named for their relatively specific ligands, i.e., kainic acid, quisaqualic acid and N-methyl-D-aspartic acid, respectively. An amino acid which activates one or more of these receptor types is referred to as an excitatory amino acid (EAA).
The NMDA subtype of excitatory amino acid receptors is activated during normal excitatory synaptic transmission in the brain. Activation of NMDA receptors under normal conditions is responsible for the phenomena of long-term potentiation, a memory-like phenomenon, at excitatory synapses. Excessive excitation of neurons occurs in epileptic seizures and it has been shown that over-activation of NMDA receptors contributes to the pathophysiology of epilepsy.
NMDA receptors are also strongly involved in nerve cell death which occurs following brain or spinal cord ischemia. Upon the occurrence of ischemic brain insults such as stroke or heart attack, an excessive release of endogenous glutamate occurs, resulting in the over-stimulation of NMDA receptors. Associated with the NMDA receptors is an ion channel. The recognition site, i.e., the NMDA receptor, is external to the ion channel. When glutamate interacts with the NMDA receptor, it causes the ion channel to open, thereby permitting a flow of cations across the cell membrane, e.g., Ca.sup.2+ and Na.sup.+ into the cell and K.sup.+ out of the cell. It is believed that this flux of ions, especially the influx of Ca.sup.2+ ions, caused by the interaction of glutamate with the NMDA receptor, plays an important role in nerve cell death. See, e.g., S. M. Rothman, et al., Trends in Neurosci., 10(7):299-302 (1987).
Agents which block responses to NMDA receptor activation therefore have therapeutic uses in the treatment of neurological disorders such as epilepsy and also in the prevention of nerve cell death resulting from hypoxia or hypoglycemia or following brain ischemia which occurs during stroke, trauma and heart attack. A number of disorders of the nervous system are associated with neurodegeneration that may be caused by overactivation of NMDA receptors. Antagonists of NMDA receptor-mediated responses have potential therefore for the treatment of such disorders as Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic Lateral Sclerosis, Down's Syndrome and Korsakoff's disease.
Research on the NMDA receptor-ion channel complex has led to determination of a receptor site within the ion channel known as the PCP receptor. See J. P. Vincent, et al., Proc. Natl. Acad. Sci. USA, 76:4678-4682 (1979); S. R. Zukin, et al., Proc. Natl. Acad. Sci. USA, 76:5372-5376 (1979); M. S. Sonders, et al., Trends in Neurosci., 11(1):37-40 (1988); and N. A. Anis, et al., Br. J. Pharmacol., 79:565-575 (1983). A compound which binds to the PCP receptor can act as an ion channel blocker, thereby interrupting the flow of ions through the cell membrane. In this manner, agents which interact with the PCP receptor act as non-competitive antagonists reducing the agonist action of glutamate at the NMDA receptor.
Known PCP receptor ligands include PCP, i.e., Phencyclidine, analogues such as 1-[1-(2-thienyl)-cyclohexyl]-piperidine (TCP), benzomorphan (sigma) opiates, and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptene-5,10-imine (i.e., the drug MK-801, see U.S. Pat. No. 4,399,141). See, also, E. H. F. Wong, et al., Proc. Natl. Acad. Sci. USA, 83:7104-7108 (1986), and W. J. Thompson, et al., J. Med. Chem., 33:789-808 (1990).